THEORY AND MARGINS

Resources - To access the Google Drive folder with the tasks and resources click here.

The Past - The diagram below shows the some major landmarks from the Earth's formation 4600 million years ago to the present day.​

​The image below is taken from a BBC webpage called 'Inside the Earth' it has five video clips that look at how the planet was formed, how the crust is divided and the structure of the earths interior - to watch these video clips click here. ​

The Structure - The video and diagram below show the structure of the earths interior - it is broken up into 4 layers - the crust, mantle, outer core and inner core. To find out more detail about each of the earths layers click here.

​The crust is solid and is the layer we live on. It is usually between 10km and 60km thick. Its thickness is often referred to as the relative thickness of an apple skin (when compared to the size of an apple).

The mantle the thickest section of the earth with a diameter of about 2900km. It is often described as being semi-molten, but in reality the top is hard rock and as you near the outer core it is beginning to melt (magma). Convection currents are found in the this layer.

The outer core layer is believed to be liquid and largely made of iron and nickel. It is extremely hot with temperatures up to 5,500 degrees centigrade.

​The inner core layer is believed to be solid, because of the immense pressure placed upon it. It contains the centre of the earth which is about 6,378km from the surface. It is also extremely hot at about 5,500 degrees centigrade.

​​The Crust - There are a few handfuls of major plates and dozens of smaller, or minor, plates. Six of the majors are named for the continents embedded within them, such as the North American, African, and Antarctic plates. Though smaller in size, the minors are no less important when it comes to shaping the Earth. The tiny Juan de Fuca plate is largely responsible for the volcanoes that dot the Pacific Northwest of the United States.

Oceanic crust, extending 5-10 kilometers (3-6 kilometers) beneath the ocean floor, is mostly composed of different types of basalts. Geologists often refer to the rocks of the oceanic crust as “sima.” Sima stands for silicate and magnesium, the most abundant minerals in oceanic crust. Oceanic crust is dense, almost 3 grams per cubic centimetre (1.7 ounces per cubic inch). Oceanic crust is constantly formed at mid-ocean ridges, where tectonic plates are tearing apart from each other. As magma that wells up from these rifts in Earth’s surface it cools, it becomes young oceanic crust. The age and density of oceanic crust increases with distance from mid-ocean ridges.

Continental crust is mostly composed of different types of granites. Geologists often refer to the rocks of the continental crust as “sial.” SialCrust stands for silicate and aluminum, the most abundant minerals in continental crust. Continental crust is much thicker as 70 kilometers (44 miles), but also slightly less dense at about 2.7 grams per cubic centimetre.

The map below show the distribution of the Earths tectonic plates and in which direction they move. To find out more information about the formation and destruction of the Earths crust click here.​

​The Age - The two maps below show the age of continental and oceanic crust that make up the different plates. Notice how the continental crust is much older than the oceanic crust.

The Movement - The plates move because of convection currents in the Earth's mantle. These are driven by the heat produced by the decay of radioactive elements and heat left over from the formation of the Earth.

Localised radioactive decay in the core produces heat which warms the material of lower mantle to produce semi-molten rock. As the semi-molten rock is less dense than the surrounding mantle it starts to rise and on reaching the underside of the crust it starts to diverge. It is the friction between the diverging convection current and the crust that causes the tectonic plates to move apart. The video and image below illustrates how convection currents move these plates.​

The two video below show how tectonic plate movement has changed the location of the UK and how the location of the continents might look in the future.​

​The Margins - Each plate is in motion relative to its neighbours, resulting in geological activity at the plate boundaries. It is also possible, though less common, for geological activity to take place in the middle of plates. There are four major types of plate boundaries; destructive, constructive, collision and conservative. The diagram below shows how the plates move, the main features and the types of hazard found at each type of plate margin. To find out more about these margins click here.​

Volcanoes - As can be seen on the map below, the earth's active volcanoes are located in specific areas. The reason for the distribution of volcanoes, is that they are located on or near tectonic plate boundaries destructive and constructive boundaries.

There are a few exceptions in the middle of the Pacific Ocean, noticeably on the islands of Hawaii. These volcanoes are caused by hotspots. The east coast of Africa is also not a plate boundary, but rather a plate (African Plate) ripping itself in half creating a rift valley lined by volcanoes.

Because volcanoes are normally found on plate boundaries their spatial concentration is limited. Their areal extent is also normally limited to areas immediately around the volcanoes, although volcanic ash clouds can potentially have global impacts by disrupting air travel and causing climate change.

Ring of fire: This is the name commonly given to the area around the Pacific Ocean. It gets its name because it has the biggest concentration of volcanoes.

Hot spots: These are volcanoes that are not found on plate boundaries. The most common explanation in mantle plume theory. This is when hot magma melts the crust above and escapes. Because tectonic plate are constantly moving, but mantle plumes stay stationary they normally create a chain of volcanoes e.g. the islands of Hawaii.

Earthquakes - An Earthquake is the sudden ground motion or vibration produced by a rapid release of stored-up energy.

The map below shows that Earthquakes tend to follow a fairly distinct pattern. The reason for their fairly organised distribution is that they tend to be focused along plate boundaries, where there are build ups and releases of pressure as plates collide or diverge. Unlike volcanoes, earthquakes can be found at any plate boundary.

There are a few exceptions to the rule like the earthquakes found in Central Australia or the east coast of the US. These exceptions are called intraplate earthquakes. These are earethquakes found in the centre of plates. The quakes might be on old fault lines or weaknesses that are often unknown. These earthquakes can cause large damage because people aren't expecting them. Humans can also cause earthquakes through explosions, the building of dams and mining.​

The largest and least frequent earthquakes tend to happen on destructive plate margins and the smallest and least frequent on constructive plate margins. The table below shows the world's highest-magnitude earthquakes since 1950. Find these on a map of the world and see which type of plate margin they were located on.

​The two sentences below explain why earthquakes occur at earthquake different types of plate margins, (destructive, collision and conservative);​'As a tectonic plate is stretched tensional stress develops within the rock and when this stress reaches a level that exceeds the strain threshold the rock suddenly fractures. This sudden movement releases energy in waves that travel through the earth's crust and cause an earthquake'.

'As tectonic plates move slowly past each other friction at the plate edge causes them to become stuck. This causes stress to develop between the two plates. When the stress between the plates exceeds the friction, the plates suddenly jerk past each other. This sudden movement releases energy in waves that travel through the earth's crust and cause earthquakes'.

Consolidation - ​The diagram below could be used to consolidate your knowledge and understanding of the the main features and processes that operate at each type of plate margin.​

​The Relationship - The diagram below shows that new oceanic crust if formed at constructive plate margins and then destroyed as it subducts into the mantle at destructive plate margins.​